The Shadow Plane

One of the four True Planes, the shadow plane is filled with creeping darkness. Like the other three True Planes, it partially overlaps with the rest of the planes while also having regions that are "pure," with no overlap.
When not mixed with other planes, the shadow plane is all but lightless: dark plains populated with amorphous shadows are illuminated by only the dimmest dark-blue light. Both the landscape and the native creatures seem to shift like black smoke, making it difficult for an outsider to explore. It is unknown to the mortals of Nexus whether any civilizations exist on the shadow plane, or if it's filled only with bestial nightmares. Most of what's commonly known about the shadow plane is inferred from its impacts on the Nexus rather than being learned straight from the source.